Robert Roos and Lisa Schnirring, writing for CIDRAP (Center for Infectious Disease Research & Policy) New have an excellent overview of the CDC's change in advice on the use of Tamiflu for seasonal influenza.
As most of my readers are aware, starting roughly a year ago we began seeing Tamiflu resistance showing up in some European seasonal flu viruses.
See Antiviral Resistance Worries For Upcoming Flu Season for earlier reports on the emergence of resistant strains.
The resistant virus is H1N1, which is only one of several circulating strains of flu viruses. The H3N2 virus, thus far, remains sensitive to the drug, as does the influenza B virus in circulation.
This resistance is caused by a mutation, known as H274Y, where a single amino acid substitution (histidine (H) to tyrosine (Y)) occurs at the neuraminidase position 274.
Since then, the number of resistant H1N1 viruses around the world has continued to rise.
Resistance in one flu strain doesn't necessarily mean that other strains will pick up resistance. Three years ago the H3N2 strain of influenza became very resistant to Amantadine, an older class of antivirals. The H1N1 virus still remains sensitive to it, however.
This overview from CIDRAP.
Robert Roos and Lisa Schnirring Staff Writers
Dec 19, 2008 (CIDRAP News) – Increased resistance to oseltamivir (Tamiflu), the leading influenza drug, has prompted federal health officials to change their advice about flu treatment, saying clinicians for now should consider using zanamivir (Relenza) or a combination of two drugs for patients suspected of having influenza A.
The Centers for Disease Control and Prevention (CDC) today said 49 of 50 influenza A/H1N1 viruses tested so far this season have shown resistance to oseltamivir. But all the isolates remained sensitive to zanamivir and to the two older flu drugs, amantadine and rimantadine.
H1N1 is one of the three viral types and subtypes included in season flu vaccine; the others are A/H3N2 and B.
"When influenza A (H1N1) virus infection or exposure is suspected, zanamivir or a combination of oseltamivir and rimantadine are more appropriate options than oseltamivir," the CDC said in today's advisory. "Local influenza surveillance data and laboratory testing can help with physician decision-making regarding the choice of antiviral agents for their patients."
But Dr. Anthony Fiore, a medical epidemiologist in the CDC's Influenza Division, said rapid flu tests don't identify the viral subtype, and many areas don't have good flu surveillance data. "This problem really does complicate deciding what kind of antiviral to use," he told CIDRAP News.
At the same time, he said US flu activity remains low so far this season, and the resistant H1N1 viruses seem no more virulent than susceptible ones. In its weekly surveillance update today, the CDC said 3 states have reported local flu activity, while 36 states have reported only sporadic cases. Another 11 states have reported no flu cases yet.
The CDC said vaccination should continue as the primary method for preventing the flu, as the oseltamivir-resistant H1N1 viruses are antigenically similar to the H1N1 strain in this year's vaccine.
CDC interim antiviral guidance for 2008-2009
Dec 19 GlaxoSmithKline press release